Factors Affecting the Success of Colony Departure by Thick-Billed Murre Chicks

Abstract

Young Thick-billed Murres leave their natal colony while still incapable of flight, jumping directly to the sea. The brief period of transition from cliff ledge to open ocean results in significant mortality. At Coats Island, Northwest Territories, Canada, we investigated the effects of predation by gulls, murre nest site characteristics, and adult murre aggression on the likelihood that chicks departed to sea. Approximately 20% of chicks that jumped from the cliff failed to depart successfully to sea. Live chicks were rarely taken by predators. Most mortality was caused by drowning, as a result of aggression by adult murres. However, chicks accompanied by defending parents usually were unaffected by aggression from other adult murres. As a result, the most important cause of chick mortality at colony departure was the separation of chick and parent before reaching the sea. Chicks landed alone because either they or their parent were knocked from the nest ledge during fights with neighbors as they moved to the jump site, or because chicks struck cliff ledges during the descent to the water. Apparently, the nearer the nest site was to the edge of the ledge, and the fewer obstructions below the jump site, the greater the departure success of chicks. Site characteristics that determine success during departure may differ from those associated with success in incubation and chick-rearing. Hence the effect of breeding site on reproduction may be incorrectly assessed if departure success is not taken into account.